1. Field of the Invention
The present invention relates to a method and apparatus for confining a radio frequency (rf) excited plasma and, more particularly, to a method and apparatus for confining an rf excited plasma to a localized, well defined area of an etchable substrate so that non-uniformities in the substrate surface may be corrected. The present invention provides a means for shaping a substrate surface with great precision. It is particularly suitable for forming thin, uniform substrate layers.
2. Description of the Prior Art
Conventional processes for shaping surfaces of substrates and, more particularly, for thinning and figuring surfaces and films, such as silicon-on-insulator (SOI), often employ such methods as mechanical polishing, grinding, sputtering, sand blasting and ion beam bombarding. Each of these prior art processes usually have substantial processing limitations. Chemomechanical processes such as thinning, polishing and flattening, are contact methods which leave contaminants that can cause subsurface damage to a substrate. Plasma assisted chemical etching methods have improved upon prior art processes such as chemomechanical thinning because such plasma processes are non-contact, thus reducing the potential for subsurface damage. Furthermore, chemomechanical processes have limited capability to correct the thickness profile of a film because the ability to change the film profile is determined by the flatness of the underlying surface. The plasma processes of the present invention, on the other hand, can locally remove material depending on the measured film thickness at that point without contacting the substrate. Thus, a non-contact etching method such as plasma etching reduces the potential for subsurface damage of the substrate.
For optical figuring, plasma assisted chemical etching processes are superior to conventional, mechanical methods because aspherical surfaces may be figured as easily as spherical surfaces; removal rates can be high so that figure generation as well as final figure error correction can be readily achieved; material removal is non-contact and does not create subsurface damage; and surface roughness is smoothed with material removal. Methods of figuring by plasma assisted chemical etching, and more particularly optical figuring, are disclosed in U.S. Pat. No. 4,668,366. This patent discloses a method and apparatus for figuring a surface by plasma assisted chemical transport by means of mounting the surface to processed on an electrode of a parallel plate rf driven reactor. The method disclosed passes a reactive gas through a chamber where an rf field is present. The method controls the removal rates of different areas of the surface by moving a relatively small surface area electrode over the surface to be processed. The time the small electrode spends at each region affects the etching of the surface at that region. However, as will be seen later, the apparatus and method of this invention lacks a means to precisely control the profile or shape of the material removal footprint. Such a means is necessary when precise error correction of surfaces is desired. The present invention provides a means to control the profile of the material removal footprint.